In addition to hydroquinone, many other compounds have been described as tanning developers. In particular, bromohydroquinone, chlorohydroquinone, methylhydroquinone and morpholino-methylhydroquinone are described as tanning developers in U.S. Pat. No. 3,364,024. Gentisic acid and its combination with a 3-pyrazolidone auxiliary developing agent are described in U.S. Pat. No. 3,300,307. Methyl gentisate, gentisamide, gentisaldehyde, .beta.-hydroxyethylgentisamide, gentysaldoxyme, phenoxyhydroquinone, toluenehydroquinone, durohydroquinone, xilohydroquinones and pseudocumohydroquinones are described as tanning developers in U.S. Pat. No. 3,143,414.
One of the main requirements of the tanning development material is that of allowing film processing with automatic processing machines which often have a high turbulence in the activation region. In this region, where the film development is activated by alkaline solutions substantially free of developers, the developers contained in the films and/or the quinones formed thereby upon development of the silver halides, can be lost in the external water solution before sufficient tanning development and formation of an irreversible and mechanically resistant gel can take place.
To overcome the problem (which is connected with the tanning rate referred to before), the use of poorly soluble and/or poorly migrating developers has been proposed (U.S. Pat. Nos. 4,283,479; 4,299,909; 3,440,049; 3,453,109; 2,751,295 and GB Pat. No. 2,044,943). Examples of such compounds are alkyl gallates, alkylcatechols and phenyl- or phenoxy-hydroquinones.
French Pat. No. 1,463,899 describes several tanning developers wherein at least a hydrogen atom of the benzene nucleus of the hydroquinone is substituted with high molecular weight alkyl groups. Examples of such developers are diisooctylhydroquinone and 2,5-dihydroxy-4-octadecyl-benzenesulfonic acid.
Even in this case, the developer usefulness is due to its poor capability of migrating which prevents its sublimation or migration while storing or processing the virgin films including it.
Some of these developers give satisfying results in materials for the reproduction and photocomposition of technical drawings or other line images, but not for realizing competitive and reliable materials in the most sophisticated sector of the Graphic Arts such as that of dot formation, where high quality images are required.
For example, in the materials containing spirobiscromanes combined or not with other co-developers deriving from catechol or gallic acid, there is still a certain difficulty in removing the non-tanned regions of the layer contained between two tanned lines particularly close to each other, which is necessary when very high resolving power, a good tone reproduction, and image edge quality are required.
R. B. Pontius (PSA Journal, Section B, September 1951, page 76, volume 17 (Phot. Sci. Tech.), divides the developers into tanning, poorly tanning and non tanning developers and mentions some cases of tanning synergism and even cases of anti-tanning synergism. Although this article points out in particular the tanning synergism, no example of synergism related to the use of at least two different hydroquinone structures is mentioned.